This invention relates to optical amplifier arrangements, which are for example provided at regular intervals along optical fibers within optical communication networks. These amplifier sites typically comprise optical nodes at which signal amplification takes place in addition to some sites at which signal routing functions are also preferred.
When long haul, regional or metro optical communication networks are installed, amplification sites are required at intervals along the optical fiber spans. These sites provide amplification to compensate for fiber losses over the preceding span. Current technologies allow a maximum span between sites of approximately 80 km. The amplification sites may also compensate for other distortions arising in the preceding span, for example chromatic dispersion or polarization mode dispersion.
When the network is initially installed, some of the amplification sites will provide only the amplification and other compensation functions, and will not be required to perform any signal routing functions. Other sites, however, will require add/drop or signal routing capability to enable signals to be branched off the main span or to be provided to the span. These routing nodes require some form of routing arrangement to allow signals to be added or removed from the fiber span. A patch panel may be used for this purpose, or other manual routing arrangement, or else a switching core may operate in a wavelength-dependent manner. A switching core is the most expensive part of the node design, and sufficient switching capability will only be provided for the current or short term expected traffic requirements. Some redundancy in the switching capability of the communication system may be factored into the design at the outset, dependent on the requirements for availability at this node.
FIG. 1 shows an example of a known node architecture at which amplification and signal routing functions can take place. For the purposes of clarity, the components required for transmission from west to east are shown in FIG. 1, although it will be appreciated that the node will in fact be arranged for bi-directional flow of traffic. The node 10 receives an incoming WDM signal from the west and amplifies this signal using a first amplifier 12. The amplified signal is provided to a dispersion slope compensation module 14. A first booster amplifier 16 prepares the signal for the signal routing part 18 of the node.
There are two amplifiers 12,16 at the input side of the node because the loss which can be sustained between two amplifiers is limited. Typically, the dispersion compensation module 14 may introduce a loss of up to 10 dB and a switching core may introduce a loss of up to 15 dB. There are, however, numerous other possible configurations for the amplifier stages.
There is a maximum power per wavelength at which light can be launched into a fiber before the non-linear distortions make the signal unusable. This limits the power of the booster 16, but the power of the booster amplifier can be increased if it is followed by a linear loss element (such as a switch). The signal routing part 18 has a wavelength de-multiplexer 20 which divides the incoming WDM signal into individual channels or groups of channels 22. These channels 22 are switched by a switching arrangement 24 which, in addition to routing signals across the node 10, also provides add and drop capability, not shown. The switching arrangement is commonly termed xe2x80x9cswitching corexe2x80x9d. The output signals 26 from the switching core 24 are provided to a bank 28 of variable optical attenuators which are provided for channel balancing. The balanced channels are then combined by a multiplexer 30 to define the output of the signal routing portion 18. This output is then amplified by a second booster amplifier 32 to define the east bound output of the node 10.
This node configuration will be well known to those skilled in the art. In such an arrangement, the switching core 24 can provide per-channel routing of signals. However, this switching core 24 is an expensive component and will not be installed at every amplifier node where this level of switching capability is not initially required. However, subsequent changes to the network may require the switching capability at a node to be upgraded. Increasing the switching capability also increases the loss of the signal routing portion 18 so that the booster amplifier 16 will also require upgrading to support the increased switching capability.
The node illustrated in FIG. 1 is shown in simplified schematic form. For clarity, FIG. 2 shows the node architecture in which the node can perform signal routing operations between three fiber spans, to the east, west and south of the node. Thus, the node architecture shown in FIG. 2 implements a Y-branch. The incoming fibers from the east, west and south each undergo amplification, dispersion compensation, first stage boosting and de-multiplexing using the same components as described in connection with FIG. 1. In the example shown in FIG. 2, each de-multiplexer 20 provides five channels on different respective wavelengths. In the example shown in FIG. 2, the switching arrangement 24 has individual switching planes 25 for each of the different wavelengths. For example, the switching plane 25a receives as input the first channel 22a from each of the multiplexers 20, and each of these channels 22a are on the same carrier frequency. This enables the switching arrangement 24 to be designed as a number of separate switching planes 25, each designed for a specific wavelength. Furthermore, the node can be arranged to add or drop signals on predetermined wavelengths by modification to one of the switching planes only. The outputs of the switching arrangement 24 are again provided to a bank 28 of variable attenuators before being combined by multiplexers 30 to form the individual east bound, west bound and south bound signals.
In order to avoid the need to provide full switching capability when a network is installed, the node architecture needs to be designed to enable upgrades to be performed. There is also a need to provide protection/duplication of equipment to enable repair or servicing of components within the node.
The inventors have firstly recognised the need to provide protection for the switching arrangement 24, as it may require servicing or repair, and it may also be desirable to change some of the switching planes, or to add new switching planes, to allow different signal routing capability. One possible way of providing this protection is shown in FIG. 3. In this arrangement, two switching cores 24a, 24b are provided. An array of splitters 40 provides the individual channels or groups of channels 22 on two different paths, each leading to a different switching arrangement 24. The outputs from the two switching arrangements 24a, 24b are then applied to a bank 42 of two-way switches which enable one or other of the inputs to be routed to the bank of variable optical attenuators 28. This arrangement enables the switching core 24 to be replaced for upgrade or maintenance and provides duplication only of the switching arrangement itself. By providing two separate paths for the different switching arrangements 24 protection is provided, but this protection is not provided for the bank 28 of attenuators, and this arrangement does not allow upgrade of the de-multiplexer 20 and multiplexer 30 without interrupting normal service.
One way to overcome these disadvantages is to provide two full signal routing portions 18a, 18b between the splitter 40 and the switch 42 as shown in FIG. 4. In this way, the whole signal routing portion 18 is protected so that the failure of any component within the signal routing portion 18 is protected and all components can be upgraded without interrupting service through the node. However, if a switching arrangement 24 is being added to a node with no switching capability, this will normally require upgrade of the first booster amplifier 16, and the arrangement of FIG. 4 requires an interruption of service to enable this upgrade.
According to a first aspect of the invention, there is provided an optical amplifier arrangement comprising:
a splitter for providing an input WDM optical signal on at least first and second output paths;
an optical amplifier for amplifying the WDM optical signal in at least one of the paths;
a signal routing arrangement for routing individual channels or groups of channels of the WDM signal within the at least one of the paths; and
a switch for selecting the signal from one of the at least first and second output paths.
In this arrangement, there are two paths between the input and output. The path in use (the xe2x80x9cat least one of the pathsxe2x80x9d) includes an optical amplifier and a switching arrangement. Whilst this path is being used, the components in the other path can be upgraded. For example, a switching arrangement may be provided in the other path, and an amplifier may also be upgraded, without disrupting service in the path in use. This enables an amplifying node within an optical communications system to be incrementally upgraded.
For example, the switching arrangement may comprise at least one wavelength-selective tap for tapping a selected wavelength channel from the WDM signal. This wavelength may be dropped or routed to a new span through a manually-provisioned patch panel. This may be upgraded in the unused path to a multiple input and multiple output switching core for automatically routing individual channels of the WDM signal. This upgrade requires the amplifier in the path to be upgraded, and this can also be done without disrupting service in the path in use. The switch is then controlled to receive signals from the upgraded path.
The two paths can be upgraded in turn, so that in the fully upgraded arrangement, a switching core is provided in each of the first and second output paths, and each switching core is provided with a wavelength division de-multiplexer at the input of the core for dividing the WDM signal into individual channels or groups of channels.
In one embodiment, the outputs of the switching cores are provided to the switch, and a wavelength division multiplexer is provided at the output of the switch. The switch thus receives individual channels, and can therefore be arranged as a switch array, which can select different channels from different paths. This means that the arrangement can provided per-channel protection.
Alternatively, each switching core can be provided with a wavelength division multiplexer at the output of the core for combining the WDM channels into an individual WDM signal, and wherein the WDM signals from the two multiplexers are provided to the switch. The switch thus switches the WDM signal, so that per-channel protection is not possible. However, by placing the multiplexer before the switch, the multiplexer in the unused path can be upgraded or replaced without disrupting service.
When each switching core is provided with a wavelength division multiplexer at the output of the core for combining the WDM channels into a WDM signal, a further amplifier may also be provided in each path for amplifying the WDM signals, the amplified WDM signals being provided to the switch. The further amplifier (which is a second booster amplifier) is then upgradeable.
In some embodiments, the output of the switch is further amplified before defining the output of the amplifier arrangement.
According to a second aspect of the invention, there is provided an optical amplifier arrangement comprising:
an amplifier section, comprising:
a splitter for providing an input WDM optical signal on at least first and second output paths; and
an optical amplifier for amplifying the WDM optical signal in at least one of the paths; and
a switching core section at the output of the amplifier section, comprising:
a de-multiplexer for dividing an amplified WDM signal into individual channels or groups of channels;
a splitter for providing the divided channels on at least third and fourth output paths;
an optical switching core in each of the third and fourth paths; and
a switch for selecting the output of one of the switching cores.
This arrangement provides the ability to upgrade the amplifier to a higher power version, thus compensating for the loss in the switch, and avoids the need to provide upgraded amplifiers in two separate paths.
The invention also provides an optical communications system comprising a plurality of nodes connected by optical transmission lines, wherein at least one node is provided with an optical amplifier arrangement according to the invention.
The invention also provides a method of upgrading an optical amplifier arrangement, comprising defining a first path between an input and an output of the amplifier arrangement, the first path including a splitter;
amplifying the signal in the first path using a first amplifier positioned downstream of the splitter and routing the amplified signal from the first path towards the output;
defining a second path between the input and the output of the amplifier arrangement, the second path including said splitter;
providing an amplifier and a channel routing device in the second path downstream of said splitter; and
routing the amplified signal from the second path towards the output.
This method allows a channel routing device to be installed or upgraded, with installation of the required booster amplifier, without disrupting operation of the arrangement. By xe2x80x9crouting a signal towards the outputxe2x80x9d is meant that the signal ultimately contributes to the output signal, but that additional operations may be carried out on the signal before it defines the output of the optical amplifier arrangement.
For example, the amplified signal may undergo further amplification before defining the output of the arrangement.
The method may further comprise upgrading the amplifier and providing or upgrading a channel routing device in the first path downstream of said splitter. In this way, both paths may be upgraded in turn, without disrupting service, so that the fully upgraded arrangement can provide 1+1 protection.
The signal is preferably routed from the first or second path towards the output by a switch.